1. Field of the Invention
This invention relates generally to ink compositions and, more particularly, this invention relates to ink compositions useful in jet printing applications, especially "drop on demand" jet printing applications.
2. Description of Related Art
Printing according to the "ink jet printing" principle, and apparatus for carrying out such printing operations, are well known in the art. In general terms, a fluid ink is forced, under pressure, through a very small orifice in a printing head.
In so called "continuous" jet printing operations, the droplets are passed through a charging area wherein individual droplets receive an electrical charge in response to a signal. The droplets then pass through an electrical field, causing a varied deflection of the individual droplets dependent on the intensity of the charge and field. The droplets are produced continuously, and are directed to the substrate to be printed or, alternatively, to a bypass gutter.
The substrate is typically of paper (coated or uncoated), glass, metal, etc.
Due to the nature of the "continuous" jet printing process, inks used therein must be conductive so as to accept a charge. Such inks are generally water based, as aqueous solutions or dispersions are easily rendered conductive. Water based inks are generally characterized as having a low viscosity (which tends to vary with temperature) and high volatility. Both characteristics can be disadvantageous.
If the viscosity of a jet printing ink is excessively low, misting and the production of stray droplets can result, which in turn result in poor print quality. High volatility of a jet printing ink can result in clogging of jet nozzles owing to evaporation.
So called "drop on demand" (DOD) systems differ from continuous jet printing systems in that ink droplets are expelled from a printing head only when required during the printing process. Since such inks need not be conductive, they have generally been formulated using primarily non-aqueous solvents such as ethylene glycols, particularly diethylene glycols, which are characterized as having low volatility. As such, ethylene glycol based inks do not evaporate quickly and, thus, have reduced clogging problems.
However, ethylene glycol solvents have a relatively high viscosity (which varies significantly with temperature) and thus require relatively more energy for production of droplets. Also, some ethylene glycol inks have to dry by sorption into paper and thus are dependent on paper characteristics.
Ethylene glycol based inks often are mixed with water in order to reduce viscosity or contain water (e.g. 10-15 wt. %) as a result of sorption from air. This can result in clogging problems since the addition of water significantly increases the evaporation rate of the ink. Also, the added water reduces the solubility of many dyes in the solvent system.
Prior water based and ethylene glycol based printing inks exhibited significant variation in viscosity with variation in temperature within the ambient range. This creates significant problems in providing proper pressurization and control for the production of uniformly sized ink droplets. If not controlled, print quality is highly variable.